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🧪 useReducer Effects ↩️

A tiny library that enables side effects with the useReducer hook

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Table of Contents

The Problem

You are using the useReducer hook to manage your state, but you need a way to perform side effects (like fetching data from an API or logging) without mixing those concern together.

The Solution

use-reducer-effect let's you define a second function next to the reducer function, where you can manage your side effects.

This way you can run your side effect logic on certain actions, while also allowing side effects to update the store by dispatching new actions.

Getting started

Install the library with

npm install --save use-reducer-effect

and import the hook into your code

import { useReducerWithSideEffects } from "use-reducer-effect";

If you already use a reducer to manage your state you will feel right at home with using them to manage side effects to.

useReducerWithSideEffects uses the same API as useReducer does, but gives you the ability to define a function which is called after your reducer function has run to run additional logic like fetching data from an API.

Defining the effect function

The effect function takes the same argument as the reducer function. state and action.

Instead of modifying the state, it is responsible for handling side effects like making http calls.

Let's assume we want to fetch from an API whenever an action called LOAD is dispatched. When the data is there, we want to feed it back into our reducer.

Note: The effect function will be called after the reducer has updated the state.

// define the reducer as usual
function reducer(state: State, action) {
  switch (action.type) {
    case ACTION_LOAD: // this action is dispatched by clicking on the button
      return { ...state, loading: true };
    case ACTION_LOAD_SUCCESSFUL: // this action is dispatched by the side effect. We'll learn how to do this in a minute.
      return { ...state, loading: false, data: action.payload };
  return state;
// define the effect function which is executed after the reducer has run
async function effect(state, action) {
  switch (action.type) {
    case ACTION_LOAD:
      const response = await fetch(
      const data = await response.json();
      return {
        payload: data
// using the hook
const MyComponent = () => {
  const [state, dispatch] = useReducerWithSideEffects(reducer, effect);

Data Flow

Data flow visualization

  1. The action LOAD is dispatched with a payload (in this case the GitHub username)
  2. The reducer function is run and updated state is returned to the component
  3. The effect function is called, which has access to the updated state and the dispatched action (LOAD)
  4. The effect function triggers an http request
  5. It dispatches the action ACTION_LOAD_SUCCESSFUL with the API response data as payload
  6. The reducer function is called again (ACTION_LOAD_SUCCESSFUL) and updates the state
  7. The effect function is called again with the new state (API response) and the action ACTION_LOAD_SUCCESSFUL. We didn't register a side effect for this action so it doesn't do anything.


useReducerWithSideEffects(reducer, effect, initialValue?, init?)

The only difference between this function and useReducer is the effect function you need to provide as the second argument.

Effect Function

function(state, action) => Promise<action|undefined>

The effect function is called with the updated state and the action that caused the state update. It needs to return a Promise (async/await works great here) with either a action if you want to update the state again (i.e. after fetching from an API) or undefined if you do not want to update the state (i.e. if you want to use a side effect for logging purposes only)



It is possible to use the higher order function createSideEffectReducer which takes the effect function as it's only argument to create a hook which provides you with the exact API signature that useReducer uses.

This way you can provide a nice abstraction for shared functionality like logging.

const useLoggingReducer = createSideEffectReducer(async (state, action) => {
  console.log("Action", action);
  console.log("Current State", state);
const MyComponent = () => {
  const [state, dispatch] = useLoggingReducer(reducer, {
    count: 0


🔗 API Call

Use the effect function to handle data fetching from an API

🔗 Logging

Use createSideEffectReducer to create a useReducer like hook that logs all actions and state changes.


This library is greatly inspired by the idea of ngrx effects, which is an awesome library if your working with Angular.


Johannes Kling
Johannes Kling

💻 📖 🤔 💡 ⚠️

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